Iron Age: Ionic-Liquid-Mediated Interfacial Charge Transfer Enables Selective CO₂ Photoreduction to Formic Acid on...

Artificial photosynthesis enables the solar-driven reduction of CO₂ in water to form formic acid, a C₁ hydrogen carrier and renewable fuel precursor. However, selective formic acid formation in aqueous media remains fundamentally limited by inefficient interfacial proton-electron coupling. Here, we show that ionic liquids (ILs) actively mediate interfacial charge transfer through ion-pair pathways that extract and stabilize photogenerated electrons and protons, thereby enabling selective CO₂ photoreduction. In this context, our iron oxide microrods exhibit high activity for formic acid production in IL-aqueous solutions under LED irradiation. Among the different ILs, 1,2-dimethyl-1-n-butyl-imidazolium 2-methylimidazolate (BMMIm.MeIm) affords the highest efficiency, achieving a yield of about 55.4 μmol (554 μmol.g^-1) of formic acid with >99% selectivity and an apparent quantum yield of 4.4%. Spectroscopic analyses (EPR, NMR, and ex situ FTIR) reveal the formation of [CO₂]•^- and imidazolium-cation radical species, confirming the direct participation of IL in charge extraction and CO₂ activation. Mössbauer spectroscopy confirmed hematite as the predominant phase and revealed an IL-induced formation of 6-9% reduced iron (Fe(0)/Fe(I)), indicating partial Fe₂O₃ reduction within the microrods. The IL creates an organized interfacial microenvironment that tunes band energetics, promotes charge separation, and stabilizes CO₂-derived intermediates, while light-induced radical signatures indicate transient interfacial charge-transfer processes that favor selective formate production. This catalytic system also demonstrates efficiency under natural sunlight, producing 27.7 μmol (277 μmol.g^-1), highlighting its adaptability and robustness. DFT calculations further reveal that IL cation-anion orientation at Fe₂O₃ surfaces modulates band energetics and promotes interfacial charge transfer.